Electroactive materials can be broadly separated into three types of materials: piezoelectric materials, elastomers between two electric plates, and ion-containing materials. Most piezoelectric materials undergo length changes of only a fraction of one percent. The movement from electroactive materials that use an elastomer between two electric plates is visible to the naked eye, however these materials use extremely high voltages, measured in the kilovolt range, and once that type of electroactive material is activated it remains static. With ion-containing electroactive materials, the material itself responds to electricity by movement that is visible to the naked eye, and as long as the electricity is on, these materials typically continue to move. The voltage requirements for ion-containing electroactive materials are much lower than elastomeric electroactive materials, typically less than 100 volts. Historically, ion-containing electroactive materials have had some drawbacks. Many of the ion-containing electroactive polymers are inherently weak materials and, typically being hydrogels, if they dry out, then they become hard, brittle, inflexible, and thus electrically unresponsive.
Applicant has previously found that copolymers comprising cross-linked networks of methacrylic acid and 2-hydroxyethyl methacrylate, (PMA-co-PHEMA) cross-linked with cross-linking agents such as ethylene glycol dimethacrylate and 1,1,1-trimethylolpropane trimethacrylate, are superior ionic electroactive materials, with tensile strengths well above the tensile strengths of other ion-containing electroactive materials found in the prior art at that time (U.S. Pat. No. 5,736,590, [1998]). A relatively small amount of electricity caused movement.
TABLE 1Strengths of some common ion-containing electroactivepolymers compared to PMA-co-PHEMA cross-linked networksMaterialTensile Strength (MPa)Poly(acrylamide) gels0.03Poly(vinyl alcohol)-co-poly(acrylic acid) gels0.23Poly(2-hydroxyethyl methacrylate)-co-0.33poly(methacrylic acid) cross-linked networks**0.28 to 0.76 MPa range for these types of materials
In 2004 and 2005, applicant developed strong electroactive materials that had pronounced responsive movement to electricity, which led to another drawback. If the electroactive material responded quickly with a lot of movement, then the electrodes often detached. If even one electrode detached, then the actuator failed. This challenge was addressed by plasma treating the electrodes to improve the polymer-metal interface, so that the electrodes and the electroactive material would work as a unit, similar to how nerves are integrated into muscle tissue. By plasma treating the electrodes, which are inserted or embedded into the electroactive material, a much better polymer-metal interface could be achieved between the embedded electrodes and the electroactive material as described in applicant's U.S. patent application Ser. No. 11/478,431 and U.S. Pat. No. 7,935,743. A good polymer-metal interface is crucial because the electroactive materials developed by applicant undergo pronounced movement. Applicant has found that by encapsulating or coating the electroactive materials, with embedded electrodes, the actuator can be free-standing, independent of submersion in an electrolytic solution as described by applicant's U.S. patent application Ser. No. 11/478,431 and U.S. Pat. No. 7,935,743.
In 2008, applicant discovered that electroactive materials and electroactive actuators described in U.S. patent application Ser. No. 12/319,804 and U.S. Pat. No. 8,088,453 that used ion-containing electroactive materials and that are produced within a defined range of cross-linking, along with other considerations, such as dilution of the monomer mix, choice of electrolyte, and the configuration of the electrodes, allowed for the preferred movements of contraction. Electroactive polymers in the prior art undergo a variety of movement. The movement of contraction is considered to be an extremely useful movement because of the similarity to movement produced by muscle tissue. U.S. Pat. No. 8,088,453 disclosed compositions of electroactive materials that undergo contraction and electrode configurations that further increase contraction in these electroactive materials and electroactive actuators. A superior method to significantly improve the polymer-metal interface was described, preferably by plasma treating the titanium metal electrodes of the actuators with nitrogen plasma, followed by oxygen plasma or treated individually with either nitrogen plasma or oxygen plasma. By encapsulating or coating the electroactive materials, with embedded electrodes, these actuators can be operational anywhere.
The advantage of this invention over the prior art is that by blending the acetate of methacrylic acid with its suitable associated cation, such as sodium ion, with methacrylic acid, and cross-linking, the final material does not need an electrolyte post-treatment step, the unnecessary anion (from electrolyte salt) is eliminated from the final material, and the final material is extremely electroactive. Because of the high electroactivity of these novel materials, other cross-linking strategies, including the use of two or more different cross-linking agents, provide for tough, highly electroactive materials.
Because these novel highly electroactive materials undergo such drastic size changes, several strategies are used to keep the actuator together in the current invention. In addition to plasma treatment, base treatment, etching, or otherwise treating the electrodes, a bending, spiral shaped, or preferably spring shaped internal embedded electrode greatly improves the durability of the actuator because the metal electrode can flex as the highly electroactive material changes its dimension. For applications where high flexibility of actuation is needed, carbon fibers, carbon weaves, and carbon felts adhere well to these novel highly electroactive materials. By placing the positive electrode inside the highly electroactive material described, and having the negative electrode slightly external to the highly electroactive material, with suitable conduction through conductive solvent, such as distilled water with a slight amount of salt present or water containing metal and other ions, such as tap water, and applying electric input, contraction occurs. When the polarity of the electrodes is reversed, expansion occurs. Contraction and expansion can be cycled repeatedly. Also, at the distal ends of the actuator, where the internal electrode enters highly electroactive material and where any attachments enter the highly electroactive material, a stronger formulation is provided at the distal ends. This tethers the electrodes and any attachments firmly into place where they enter the highly electroactive material of the highly electroactive actuator.
The coating of the highly electroactive actuator in the instant invention can be a bilayer coating, where the inner layer is flexible and conductive so also serves as the slightly external electrode, and the outer layer serves to retain moisture of the electroactive material inside, allowing these actuator to be operational anywhere.
The coating of the highly electroactive actuator in the instant invention can be a trilayer coating, where the inner most layer can be used to force actuation in a desired direction, for example, for a linear push-pull actuator. The most inner layer is somewhat stiff and rigid, restricting motion in one plane thus maximizing motion in the desired direction for actuation. This inner layer also helps to retain the integrity of the highly electroactive material of the highly electroactive actuator. This most inner layer of the trilayer coating has small holes or is porous to allow conductive solvent to the middle layer. The middle layer of the trilayer coating serves as the negative electrode. The outer layer of the trilayer coating is elastomeric and helps to retain moisture and integrity of the highly electroactive material of the highly electroactive actuator, allowing the actuator to be operational anywhere.
For applications where these novel highly electroactive materials and actuators are subjected to cold environments, a small amount of antifreeze, such as glycerol or diethylene glycol, can be added to the solvent. The solvent is preferably water.
The instant invention may revolutionalize robots and prostheses by providing highly electroactive materials and highly electroactive actuators that have smooth two and three dimensional range of motion, good durability, high strength, and that may operate over a wide variety of environmental conditions. The degree of contraction, and expansion when the polarity is reversed, can be controlled by the voltage level of the electric input, so biofeedback could easily be linked in using these materials and actuators.